[clang] [Clang][C++26] Implement P3865R1 - CTAD for type template template parameters (PR #191409)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt commented:

:-/

I thought I could review this but it turns out I'm too tired to do anything useful, so instead all I've done is complain about style #SkilledReview

https://github.com/llvm/llvm-project/pull/191409